1. Field of the Invention
This invention relates to a thin disk gain medium for a laser and more particularly to a thin disk gain medium with an anisotropic thermal expansion.
2. Description of Related Art
Different thermal expansion coefficients impose a severe problem for bonded structures as the mechanical stress that is induced through unequal expansion/contraction during thermal changes can break the bond. For thin materials non-uniform expansion can also induce fracture during coating processes that tend to heat the substrate. In this context “bonding” therefore includes coating the first material with a second material or multiple layers, e.g. depositing a dielectric coating on a thin laser crystal. It also includes soldering a thin laser crystal to a heat sink using a third material like indium during which process the structure is heated above the melting point of said third material and cooled to form a solid bond. Thermal fracture imposes a particular problem for double-tungstate laser materials such as Yb:KYW, Yb:KGW, KYbW, Nd:KGW, Nd:KYW, when these materials are to be used in a thin disk geometry. The thermal expansion coefficient of these materials is known to be highly anisotropic.
Of the double-tungstates, the ytterbium-doped crystals hold particular promise for diode pumped solid-state lasers, both for the generation of high-power laser radiation and the generation of ultrafast pulses [Brunner et al., Optics Letters, Volume 27, Issue 13, 1162–1164, July 2002].
Yb:KYW thin disk lasers have used disks that were pressure bonded to copper heat sinks using relatively thick indium foil as a third material that liquefies under pressure. This bonding method has previously been used also for bonding Yb:YAG disks to copper heat sinks, and the relatively soft, thick indium layer can compensate for some of the thermal expansion mismatch between YAG and copper that would otherwise cause problems when heat is deposited in the disk via laser radiation.
The drawback of using such a thick indium layer is that the thermal conductivity is low, such that the temperature of the disk will always be significantly higher than that of the (cooled) heat sink, compromising the efficiency of quasi-three-level laser materials and possibly leading to thermal lensing, excessive deformation, and ultimately stress induced fracture. Solutions to solder Yb:YAG with a much thinner layer of indium using a temperature on the order of 160° C. have required the replacement of copper with a copper-tungsten alloy that is expansion matched to Yb:YAG. This method has previously failed for double-tungstates because of their large anisotropy of thermal expansion.
Previous attempts to use such materials in thin disk lasers were limited by the strong expansion mismatch within the plane of the thin disk, which caused thermal fracture either during the coating of the disk, or later during the bonding of the disk, or later during laser operation. According to Pujol et al., J. Appl. Cryst. 35, 108–112 (2002) the diagonalized linear thermal expansion tensor of KYbW has the following expansion coefficients along the principal thermal axes: 2.57×10−6/K, 8.72×10−6/K, and 16.68×10−6/K.
Different values for the thermal expansions coefficient are given by Aus der Au, (Hartung Gorre, 2001, ISBN 3-89649-636-0), p. 32. In contrast to the above data, the values for Yb:KGW are 4.0×10−6/K, 3.6×10−6/K, and 8.5×10−6/K and are along the crystallographic axes a, b, and c, respectively. Using this data, Brunner et al. teach to use a b-cut crystal for spectroscopic reasons rather than for thermo-mechanical properties: “We chose the laser polarization to be parallel to the a axis of the b-cut Yb:KYW crystal to utilize the largest emission cross section.” Note that according to these published values, the thermal expansion along one of the two axes in the bonding plane is at least 2.25 times larger than the other.
There is a need for an improved optical system, and its methods of use, that has a thin disk gain media. There is a further need for an optical system, and its methods of use, that has a thin disk gain media with an essentially constant thermal expansion coefficient in all directions in a plane of a cooling surface of the thin disk gain media.